(1) Field of the Invention
The present invention relates to a sonar quieting system which utilizes a gaseous cavity to reduce hydrodynamic noise associated with turbulent boundary layers and turbulent wakes of high speed operations.
(2) Description of the Prior Art
It is a requirement of some current naval operations to operate high frequency mine-hunting sonar systems towed from high speed surface craft. These craft can operate at speeds exceeding 30 knots but the craft produce bubbly wakes (high frequency noise source) and generate high propulsion noise. Sonar systems towed in the wake of high speed surface craft are thus adversely affected by the generation of background noise by the craft. As such, the sonar systems are limited in their effective detection range.
A similar problem exists for future fast transport ships. Some concepts have been proposed in which the transport ships can operate at speeds up to and exceeding 100 knots. However, the ability of the ships to maneuver at their design speeds is limited. Consequently, the ability to detect obstacles at significant ranges thereby increases the ability of the ships to avoid collisions with marine mammals, mines, and assorted debris.
Sonar systems towed at very high speeds are affected by noise sources which may be controllable. In a first example, the turbulent flow of water over the streamlined firing of a sonar array generates pressure fluctuations on the firing. Both turbulent boundary layers and turbulent wakes contribute to this type of structural excitation of the sonar array. The pressure fluctuations can be experienced directly on the sonar array when the flow over the array is turbulent, or indirectly as the pressure fluctuations away from the sensor face are transmitted through the structure. In another example, cavitation bubbles and collapsing vapor bubbles can also produce large structural excitations.
A preferred method of control is to maintain laminar flow over the array face which minimizes hydrodynamic noise and acts to physically isolate the array face from portions of the structure experiencing large pressure fluctuations.
The propulsion system of the vessel or craft is a large producer of noise. Blade tonals, cavitation bubbles, and entrained air all produce noise which can propagate through the moving marine environment to the sonar array. Similarly, breaking bow-waves, hull slapping, ship machinery noise, and other ship related noise sources can reach the array through the marine environment. Isolating the array from these sources by significantly reducing or eliminating the direct acoustic path between the source and the array would greatly improve the array performance.
As a result, there is a need to isolate a forward-looking sonar array from own-ship and wake noise and to minimize hydrodynamic noise resulting from turbulent surface pressure fluctuations.
Accordingly, it is a general purpose and primary object of the present invention to provide a sonar quieting system which isolates a forward-looking sonar array from the propulsion noises of a tow ship or craft.
It is a further object of the present invention to provide a sonar quieting system which minimizes hydrodynamic noise resulting from turbulent surface pressure fluctuations.
To obtain the objects described, there is provided a sonar quieting system comprising a cavitator for forming an envelope of gas, means for supporting the cavitator to a marine platform, a forward-looking sonar array mounted to a forward face of the cavitator, and means for pumping a gas into the envelope to create a gas cavity capable of enveloping the supporting means and equipment downstream of the cavitator.
Another aspect of the present invention is a method for reducing hydrodynamic noise associated with turbulent boundary layers and turbulent wakes thereby enabling high speed operation of the forward-looking sonar array. The method comprises the steps of moving the marine vessel through water at a speed sufficient for the cavitator to create an envelope of gas, and injecting a fluid into a region aft of the cavitator to create a vapor shield between the sonar elements and any acoustic sources aft of the sonar elements.
Other details of the sonar quieting system, as well as other objects and advantages attendant thereto, are set forth in the following detailed description and the accompanying drawings wherein like reference numerals depict like elements.